(1) Field of the Invention
The present invention relates to a marine tow cable and more specifically to a marine tow cable incorporating therein both a coaxial electronic communications cable and one or more fiber optic transmitter elements. Such cables are commonly referred to as electrical-optical-mechanical (EOM) tow cables. EOM tow cables are in common use by surface and/or undersea watercraft to tow scientific data sensing and/or recording instrument packages or military surveillance and/or enemy detection equipment. The instrument packages not only require that the cable withstand varying tensile forces imparted thereto by the hydrodynamic drag of the instrument package vehicle under tow but also incorporate a secure communication link with the towing craft.
(2) Description of the Prior Art
Prior art EOM cables generally position the fiber optic transmitters about the axial center line of the cable and surrounded by a protective metal tube thereby requiring that the core conductors, or the electronic coax, be placed outside the metal tube. This type of structure does not provide for a minimum cable diameter as will be further appreciated herein below.
Because glass fibers are by nature almost inextensible, it is undesirable to orient optical fibers axially within a cable, such as an EOM cable, wherein the cable is constantly subjected to cyclic stretching and contracting forces imparted thereto during towing operations. As the elastic components of the tow cable stretch, under towing forces, the optic fibers will be subjected to tensile loads. Although glass fibers exhibit great mechanical strength properties when in tension, because they are less extensible than the other cable components, glass fibers will catastrophically fail, without warning, upon reaching their extension limit. Further, under compressive loading, their performance is degraded by buckling and microbending. Thus, as the tow cable is wound upon a storage drum and/or passes over guide pulleys while being wound in or out, bending forces will be imparted to the tow cable thereby subjecting any optical fibers on the inside bend radius, or compressed side of the curved cable, to undesirable compressive forces and possible buckling failure.